This invention relates to a method for treating high strength low alloy steel whereby a material having markedly improved formability is provided which after forming and aging has a yield strength and tensile strength substantially equal to or higher than the original values.
Plain carbon steel having a yield strength of 30 to 40 ksi was used extensively in early automobiles and is presently the most commonly used automotive structural material. However, in recent years the need to satisfy safety and emission requirements resulted in progressively increased vehicle weight. At the present time there is an urgent need to conserve materials and energy. Structural vehicle material may be conserved and vehicle weight reduced by developing and using structural materials having a higher strength to weight ratio. One of the more promising potential substitute materials for the low carbon steel is the family of high strength low alloy (HSLA) steels, SAE 950X and SAE 980X, which have yield strengths in the range of 50 and 80 ksi, respectively. These are relatively new steels and have a chemistry which is similar to that of the plain carbon steel. Their superior strength is achieved by a controlled hot rolling schedule and a rapid controlled cooling which produces a very small ferrite grain size. Further, by minor additions of suitable alloying elements such as vanadium, niobium or titanium, which are good carbide and nitride formers, additional strength is achieved by the mechanism of precipitation hardening and solid solution strengthening. To insure isotropic properties, small quantities of rare earth elements or zirconium are added to control the shape of sulfide inclusions; small globular sulfides are prevented from elongating into stringers during hot rolling.
The HSLA steels have high strength, fair ductility, some directionality and, because of a low carbon equivalent, good weldability, but their formability is inferior to that of hot rolled plain carbon steels for all methods of sheet metal forming. The poor formability of the SAE 980X steels, for example, is one of the principal reasons for their limited use in automotive applications. To the extent that these steels are useable, their higher strength can result in excessive wear of tools and dies.